The statements in this section merely provide background information related to the present invention and may or may not constitute prior art.
A typical automatic transmission includes a hydraulic control system that, among other functions, is employed to actuate a plurality of torque transmitting devices. These torque transmitting devices may be, for example, friction clutches and brakes. The conventional hydraulic control system typically includes a main pump that provides a pressurized fluid, such as oil, to a plurality of valves and solenoids within a valve body. The main pump is driven by the engine of the motor vehicle. The valves and solenoids are operable to direct the pressurized hydraulic fluid through a hydraulic fluid circuit to the plurality of torque transmitting devices within the transmission. The pressurized hydraulic fluid delivered to the torque transmitting devices is used to engage or disengage the devices in order to obtain different gear ratios.
In certain transmission configurations, actuation of the torque transmitting devices is achieved by selective release of an accumulator charged with hydraulic fluid. The accumulator is charged by the main pump. However, the accumulator typically requires intermittent charging, and therefore it is not necessary to have the main pump constantly operating. Constantly operating the main pump increases the torque load on the engine and reduces fuel efficiency of the motor vehicle. A main pump driven by the engine, however, cannot be selectively disengaged. One solution is to use an electric pump that can be selectively turned off when the accumulator is fully charged. While electric pumps are effective, there is room in the art for alternative solutions that reduce the amount of complex and expensive components while improving efficiency and controllability of the system during operation of the motor vehicle.